GW170817: Limits on D>4 Spacetime Dimensions

[fresh_42]

I found this article about a paper on https://phys.org/news/2018-09-gravitational-dose-reality-extra-dimensions.html which I find interesting in his own respect, but especially for the fact that the GW experiments can actually reveal insights and that good old GR/SR is again at least supported.

Limits on the number of spacetime dimensions from GW170817
Kris Pardo, Maya Fishbach, Daniel E. Holz and David N. Spergel

Published 23 July 2018 • © 2018 IOP Publishing Ltd and Sissa Medialab
Journal of Cosmology and Astroparticle Physics, Volume 2018, July 2018

http://iopscience.iop.org/article/10.1088/1475-7516/2018/07/048/pdf

Abstract
The observation of GW170817 in both gravitational and electromagnetic waves provides a number of unique tests of general relativity. One question we can answer with this event is: do large-wavelength gravitational waves and short-frequency photons experience the same number of spacetime dimensions? In models that include additional non-compact spacetime dimensions, as the gravitational waves propagate, they "leak" into the extra dimensions, leading to a reduction in the amplitude of the observed gravitational waves, and a commensurate systematic error in the inferred distance to the gravitational wave source. Electromagnetic waves would remain unaffected. We compare the inferred distance to GW170817 from the observation of gravitational waves, dLGW, with the inferred distance to the electromagnetic counterpart NGC 4993, dLEM. We constrain dLGW = (dLEM/Mpc)γ with γ = 1.01+0.04−0.05 (for the SHoES value of H0) or γ = 0.99+0.03−0.05 (for the Planck value of H0), where all values are MAP and minimal 68% credible intervals. These constraints imply that gravitational waves propagate in D=3+1 spacetime dimensions, as expected in general relativity. In particular, we find that D = 4.02+0.07−0.10 (SHoES) and D = 3.98+0.07−0.09 (Planck). Furthermore, we place limits on the screening scale for theories with D>4 spacetime dimensions, finding that the screening scale must be greater than ~ 20 Mpc. We also place a lower limit on the lifetime of the graviton of t > 4.50 × 108 yr.

 

[kimbyd]

First, here's the preprint link, so people can view the whole paper without a subscription:
https://arxiv.org/abs/1801.08160

It's a neat idea, but I'm worried the basis for the argument is incorrect (for some background: https://en.wikipedia.org/wiki/Large_extra_dimension). The fundamental issue is that measurements based on gravitational attraction would be far more sensitive to this kind of model: any "leakage" of gravity waves would also imply that gravity would start falling off more rapidly at larger distances.

The problem is that "large" extra dimension theories work exact opposite to this: the gravitational attraction falls off rapidly not at large scales, but at small scales. The idea is that the "large" extra dimension is not large at all in human terms. It's just large compared to the Planck scale. One model, for example, has the "large" extra dimension is only one femtometer in size (one millionth of one billionth of a meter). At distances larger than that, gravity falls off as 1/r^2 just like everything else. On really small scales, though, it falls off more rapidly, which means that the gravitational constant G we measure ends up only being a fraction of the "true" G.

Gravity waves measured from far away are completely unaffected. To visualize why this is, imagine that our universe is a plane with some amount of thickness, and we only inhabit the exact center of the plane. The gravity waves, as they spread out, will fill the entire volume, but are still constrained to this narrow region. So we only ever measure a fraction of the gravity waves, but they don't "leak" because they're still constrained to be within 1 femtometer of the reality we can measure.

Unfortunately I think this paper is just looking at the wrong thing. Of course it finds no evidence for leakage, because there wouldn't be in any model that fits our current understanding evidence of how gravity works.

 

[kimbyd]

Follow-up:

One interesting aspect of this is that this measurement might be used to constrain modified gravity theories which attempt to explain dark energy without a cosmological constant. This measurement indicates that the 1/r^2 behavior of gravity holds across Mpc scales. This might also constrain modified gravity theories which attempt to explain dark matter.